Aircraft: B787
Phase of flight: Climb
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor

On the climb out of ZZZZ the FA's called and said there was a burning smell in the back of the AC. They also reported seeing smoke. The Captain asked the other IRO to go back and assess the situation. Approximately 5 minutes later he calls the flight deck and said there was a burning smell but he suspected it was an oven and was not concerned. The captain asked me to go back and see what I thought. When I got to the back of the jet I could smell a burning smell as well. I did not see any smoke but the FA's said they did and used the [urgent] words with me. They were concerned; did not feel safe and wanted to return. I called the captain on the interphone and told him what I had observed; smelled as well as what the FA's had voiced to me as far as their concern. He said we would troubleshoot and continue on. When I told the FA's this they were not happy and started calling the Captain and again telling him they were concerned; did not feel safe; and wanted to return. He told them the same thing that he told me.When I returned to the flight deck I asked the Captain what he thought about returning to ZZZZ. He told me he could 'find nothing wrong with his airplane.' At this point I was not comfortable continuing on a 13 hour ETOPS flight with an unknown burning smell in the back of the airplane. The decision was then made to contact Maintenance Control and get their opinion on the next course of action. They suspected what we did that it could be a galley oven malfunction but could not be 100% sure that was the problem. They agreed that the safest course of action would be to return. At this point the decision was made to return. Next; we began the diversion process and ran all appropriate checklists and followed all diversion procedures. We were going to land overweight so we had to also jettison approximately 100;000 lbs of fuel. We had a problem with the auto fuel balance system and ran the appropriate checklist for that as well. We ended up making it back safely with no injuries or substantial damage to the airplane. The mechanics suspected it was a heating element malfunction in the aft galley oven but had not concluded that with 100% certainty when we left the airplane and headed back to our hotel. During this flight I was the flying pilot (FP). During departure for an ETOPS flight; while transitioning from climb to cruise; the cabin called the flight deck to report an electrical issue with an oven in the aft galley. The report from the flight attendant was that when the oven #7 was switched on there was a malfunction; FAIL was on the status message on the face of the oven and there was an 'electrical smell' in the galley. The flight attendant reported seeing a brief moment of smoke. The purser responded to the area and removed power to oven #7 by pulling the CB. There was a brief activation of the smoke warning system with a tone and flashing lights. An IRO was sent by the captain to assist the flight attendants in evaluating the issue. With the full flight crew and purser; a sat phone call was initiated with dispatch and Maintenance Control. All information listed above was discussed. The purser also reported that 5 of the flight attendants did not feel comfortable to continue and felt there was a continued safety issue because of the 'ozone smell' in the galley area. Maintenance Control recommended a return. The captain; using great CRM; listened to everyone's concerns and thoughts; along with dispatch and Maintenance Control made the decision to return. Because the aircraft was overweight for landing; a full jettison was accomplished: for a total of 100;000 pounds jettisoned and the flight returned to the departure airport where a normal approach and landing was accomplished. Fire equipment followed aircraft once on the ground during taxi and reported no abnormalities. B787 First Officers were informed by the cabin crew that there was smoke and fumes in the aft galley apparently from a failed oven. The oven was switched off and the circuit breaker was pulled; but the flight attendants wanted to return to the departure airport. However; the Captain believed the problem was solved. Maintenance Control was contacted and after some deliberation the flight returned to the departure airport.